1. Field of the Invention
The present invention relates to a charge injection type light emitting device, and more particularly to an organic, charge injection type light emitting device having an organic active layer.
2. Related Background Art
An organic light emitting device is a device in which a thin film containing a fluorescent organic compound is interposed between an anode and a cathode; excitons of the fluorescent compound are generated by injection of electrons and holes from the electrodes and a light radiated when the excitons return to the ground state is utilized.
In a research by Eastman Kodak Company in 1987 (Appl. Phys. Lett. 51, 913 (1987)), there was reported a light emission of about 1,000 cd/m2 at an applied voltage of about 10 V for a device of functionally separated two-layer structure using ITO for an anode and a magnesium/silver alloy for a cathode, respectively, an aluminum-quinolinol complex as an electron transporting material and a light emitting material and a triphenylamine derivative as a hole transporting material.
In addition, light emission of from ultraviolet region to infrared region is possible by changing the type of the fluorescent organic compound and researches of various compounds have been conducted actively recently.
Moreover, other than the organic light emitting device using the low-molecular materials as described above, a group of Cambridge University has reported an organic light emitting device using a conjugated polymer (Nature, 347, 539 (1990)). In this report, light emission in a monolayer is confirmed by forming a film of polyphenylenevinylene (PPV) in a coating system.
Thus, recent progress in organic light emitting devices is remarkable, and possibilities for a wide range of applications are indicated since it is characterized in that a thin and lightweight light emitting device having a high luminance at a low applied-voltage, diversity of light emitting wavelength and high-speed response can be prepared.
However, a higher-luminance or higher conversion efficiency light output is required under present circumstances.
Conventional organic light emitting devices, in the case of using luminescence by charge injection excitation, have been said to have an upper limit of 25% on the internal quantum efficiency. Considering the loss by internal reflection at the interface of the transparent substrate of the device and air, this value is considered to be equivalent to 5% as external quantum efficiency. Although singlet excited states and triplet excited states of light-emitting molecules arise at random in the case of charge injection excitation, the singlet excited states and triplet excited states are supposed to be generated at a ratio of 1:3 considering that the multiplicity of singlet excited state is 1 while the multiplicity of triplet excited state is 3. Since the ground state is a singlet state in common molecules, the triplet excited state has a very small probability to effect transition to the ground state and therefore does not contribute to luminescence at ordinary temperature. For this reason, it has been said that the upper limit of internal quantum efficiency will be 25% even if all the singlet excited states effect transition.
Recently, in an attempt to overcome this limit, a material has been proposed which utilizes the heavy atom effect and remarkably increases the transition probability from the triplet excited state to the ground state and thereby enables triplet luminescence of a sufficiently high efficiency even at room temperature. M. A. Baldo et al. reported that use of an organic iridium complex as a light-emitting molecule (or emissive molecule) which shows efficient phosphorescence luminescence from the triplet excited state leads to 8.0% of external quantum efficiency (equivalent to about 40.0% of internal quantum efficiency)(Reference 1: Baldo M. A., Lamansky S., Burrows P. E., Thompson M. E., Forrest S. R., “Very high-efficiency green organic light emitting devices based on electrophosphorescence”, Appl. Phys. Lett., vol. 75, No. 1, pp 4–6 (1999).
Although searches for materials that exhibit efficient phosphorescence luminescence using the heavy atom effect have been performed actively thereafter, many of the materials other than iridium complex have a problem in respect of efficiency, stability, etc. and, at present, there is no material of which practical use is in prospect. Meanwhile, iridium complex that uses iridium, relatively rare element, is expensive and has a problem from an economical point of view. Furthermore, although the luminescence is efficient, it is phosphorescence luminescence and has a difficulty that when it comes to a high luminance range, triplet—triplet annihilation will occur and the light-emitting quantum efficiency falls. Moreover, design flexibility in terms of luminescence color is also restricted due to the limitation by an iridium complex.
Particularly from this viewpoint of increasing design flexibility of luminescence color, there has been suggested that a singlet luminescent material is allowed to coexist in an emissive layer in combination with an iridium complex so as to make use of energy transfer from the iridium complex to effect light emission of the singlet luminescent material (Reference 2: Japanese Patent Application Laid-Open No. 2002-50483). In this suggestion, a singlet luminescent material to be combined with the iridium complex is said to be selected from materials which are easy to effect reverse intersystem crossing, that is, intersystem crossing from a triplet excited state to a singlet excited state, in order to carry out energy transfer efficiently from the triplet excited state of the iridium complex.
In spite of such an effort, however, the fact that an iridium complex is used remains unchanged, and therefore, disadvantages associated with expensiveness of an iridium complex and triplet—triplet annihilation which leads to a low luminescence efficiency in the high luminance range, are not solved.